In the manufacture of semiconductor devices such as microwave transistors and silicon integrated circuits, shallow phosphorus diffusion in semiconductor silicon has become important. The characterization of semiconductor bodies is influenced substantially by diffusion profiles, especially from the emitter of a n-p-n structure, and the profiles are further dependent upon the diffusion source used. Up to the present time, liquid diffusion sources have been chiefly utilized in the diffusion process since satisfactory solid phosphorus diffusion sources have not been widely available. The liquid sources which have been employed are compounds such as phosphine (PH.sub.3), phosphorus pentoxide (P.sub.2 O.sub.5), phosphorus oxychloride (POCl.sub.3) and phosphorus chlorides (PCl.sub.3 and PCl.sub.5). Of these liquid sources, POCl.sub.3 and PH.sub.3 have most frequently been used. These five phosphorus compounds are all low melting-point substances and have liquid or gas phases at temperatures below 650.degree.C.
Conventional doping methods for phosphorus diffusion as performed with liquid diffusion sources are, briefly, as follows. One of the compounds listed above is heated at a low temperature, below 650.degree.C, and the phosphorus gas and/or phosphorus compound gas thus developed is introduced into a doping chamber kept at a high temperature, ranging from 850.degree.C to 1200.degree.C. In this chamber the silicon wafers to be doped are arranged perpendicular to the phosphorus gas flow. In this method, the carrier concentration of phosphorus, p-n junction depth, and other electronic properties of the doped wafer are primarily influenced by the reaction condition between phosphorus gas and the solid silicon wafer. This reaction is further influenced by the flow rate of gas. When a uniform diffusion layer is required, a uniform flow of gas is necessary, which is quite difficult to establish. As a result, uniform diffusion of phosphorus in terms of each silicon wafer is difficult to control. This is one of the shortcomings of conventional phosphorus doping methods using liquid diffusion sources. Another deficiency of the liquid diffusion source method is inconvenience due to the dangerous nature of the liquid sources. Phosphine, phosphorus oxychloride and many other phosphorus compounds are toxic, corrosive, flammable or explosive.
While liquid diffusion sources continue to be used for the treatment or doping of semiconductor materials, the disadvantages of irregular diffusion control and high toxicity must be overcome to give a satisfactory diffusion procedure. An effective phosphorus diffusion or doping procedure for semiconductor silicon should provide: (1) a shallow phosphorus doping in silicon; (2) the doping procedure should not be complicated and should have a high reproducibility and reliability; (3) the doping procedure should be safe, even if personnel are exposed to exhaust gas during doping; (4) the diffusion sources should be economically reusable for many doping runs; and (5) a solid form of dopant source material should be utilized for greater safety, ease of handling, and convenience in use.
A number of solid diffusion sources have been developed in the past. Examples of such sources are indicated by U.S. Pat. No. 3,540,951, issued Nov. 17, 1970; U.S. Pat. No. 3,473,980, issued Oct. 21, 1969; and copending U.S. patent applications Ser. No. 239,897, filed Mar. 31, 1972, by McMurtry and Murata, now U.S. Pat. No. 3,849,344; and Ser. No. 374,706, filed June 28, 1973, by Murata and McMurtry, now U.S. Pat. No. 3,852,086.
In addition, prior doping techniques have included application of a doping or donor composition directly to the surface of a semiconducting material. Examples of these techniques include U.S. Pat. No. 3,514,348, issued May 26, 1970; U.S. Pat. No. 3,630,793, issued Dec. 28, 1971; U.S. Pat. No. 3,354,005, issued Nov. 21, 1967; and U.S. Pat. No. 2,794,846, issued June 4, 1957. Such techniques have suffered from a number of faults, including non-uniformity of doping and difficulty of control of dopant concentrations and junction depth.
The method of the present invention provides solid phosphorus diffusion sources, which are non-toxic, convenient, uniform, and which may be used in a standard dopant diffusion apparatus to give precise control of the diffusion treatment of semiconductor materials.